In conventional systems, as shown in FIG. 6, there has been known an elastic coupling 51 in which connection elements 52 connected to a drive side yoke in a rotation drive system of a motor vehicle and connection elements 53 connected to a driven side yoke are alternately arranged on a circumference, cords 54 and 55 are wound around every mutually adjacent connection elements 52 and 53 so as to form a band shape, and the connection elements 52 and 53 and the cords 54 and 55 are coated by an elastic material 56 such as a rubber or the like (for example, refer to Japanese Patent Publication No. 7-33852).
However, in this conventional elastic coupling 51, a two folded yarn (total of about 3000 tex), obtained by bundling two single yarns 57 having about 15000 tex as shown in FIG. 7, is used as the cords 54 and 55. In the two folded yarn, both of a number of upper twists and number of lower twists are about 30 to 40 times per 10 cm, a thickness of the cords 54 and 55 in this state of two folded yarn is about φ0.7 mm, and a cross sectional shape thereof is substantially a circular cross section.
Accordingly, when winding the cords 54 and 55 having the thickness and the cross sectional shape mentioned above around outer peripheries of the connection elements 52 and 53 several times over, minute gaps 58 are formed between the cords 54 and 55, as shown in FIG. 8, a thick winding tends to be partly generated unless the cords are wound by an accurate feeding amount (pitch), whereby a displacement (an arrow C) of the cords 54 and 55 tends to be generated at a time of winding.
Further, when using the structure in the state mentioned above as it is, a particular wound portion digs into an inner peripheral side by a tensile force applied to the cords 54 and 55 at a time of transmitting a torque (an arrow D), so that a dispersion is generated in a magnitude of the tensile force between an inner peripheral side and an outer peripheral side as well as a slack is generated in the wound portion in the outer peripheral side. Accordingly, there is a case that a durability of the cords 54 and 55 is reduced for this cause. Further, since no slack is generated in inner peripheral portions which are in contact with cords 54 and 55 wound around the connection elements 52 and 53, a disconnection is early generated, the disconnected portion gradually expands to the outer peripheral side, and there is a case that the cords run into a breakage.
Further, since the two folded yarn formed by further bundling the single yarns (bundles of filaments) with each other is conventionally used as mentioned above, each of the filaments is arranged with a comparatively large angle with respect to a torque transmitting direction. However, it is preferable in strength of the filament that a force is applied to each of the filaments in a state in which each of the filaments is arranged in parallel to the torque transmitting direction, and by so doing, a torque transmission rate is improved. In regard to this point, in the case of the conventional two folded yarn mentioned above, since the overlapped filaments of the two folded yarns are rubbed with each other with an angle, there can be considered that strength of the filament is reduced and durability of the filament is reduced.
The present invention is made by taking the above points into consideration, and an object of the present invention is to provide an elastic coupling which can improve a durability of a cord wound around a connection element, and can downsize a cord wound portion.